stellar evolution
interstellar medium (ISM)
= a star is born out of gas and dust that exists between the stars
~~ 99% of the ISM composed of interstellar gas, and of its mass about 75% from hydrogen, 25% as helium
interstellar gas consists partly of neutral atoms, molecules and charged particles (ions, electrons)
found in two terms:
cold clouds of neutral atomic / molecular hydrogen
hot ionized hydrogen near hot young gas
main sequence
name of for a continuous and distinctive band of stars that appear on a plot of stellar colour vs brightness.
the more the massive the stars, the shorter its lifespan on the main sequence
divided into upper and lower parts based on the processes that stars use to generate energy
lower mass (below 1.5 times mass of the sun)= proton-proton chain
more than 1.5 mass of the sun (upper main sequence)= nuclear fusion process can instead use atoms of carbon, nitrogen, oxygen as intermediaries in the production of helium from hydrogen atoms.
temperature gradient between the core of a star and its surface= energy steadily transported upward through the intervening layers until it is radiated away at the photosphere
convection
radiation
tends to occur in a regions with steeper temperature gradients higher opacity or both
when convection occurs in the core region it acts to stir up the helium ashes, thus maintaining the proportion of fuel needed for fusion to occur
post-main sequence
a star remains on the main sequence as long as there is hydrogen in its core that it can fuse into helium
as main sequence star ages its luminosity increases slightly, resulting in it expanding and its outer layer cooling. this explains why the main sequence is a broad band rather than a narrow line -stars move up and to the right on this band as they age
expt: our sun
move off the main sequence and up the red giant branch - fusing hydrogen into helium in hydrogran shell burning
a very short helium flash sees the start of helium core fusion and the star moves along the horizontal giant branch
eventually hydrogen core runs out and fusion stops, shutting off the outward radiation pressure
inward gravitational attraction causes the helium core to contract, converting gravitational potential energy into thermal energy
rise in temperature heats up the shell of hydrogen surrounding the core until it is hot enough to start hydrogen fusion, producing more energy than when it was a main sequence star.
this so called shell-burning causes some interesting effects.
the new increased radiation pressure actually causes the puter layer of the star to expand to maintain the pressure gradient
the expansion and cooling causes the effective temperature to drop
convection transport the energy to the outer layer of the stars from the shell burning region.
during this expansion, the star will move up and to the right on the HR diagram along the red giant branch. a G(V)- class star may end up as a high-K or low-M luminosity class 3 giant.
red giants
displays extremes of density
weakly held by gravitational force to the rest of the star and easily ejected
mass loss from a giant is typically about 10^-7 solar masses per yrs, compared with only 10^-17 solar masses per yrs currently for the sun
helium burning and the helium flash
hydrogen fusion in the shell produces more helium
this gets dumped in onto the core adding to its mass, causing it to heat up even more
when the core temperature reaches 100 mill K , the helium nuclei now have sufficient KE to overcome the strong coulombic repulsion and fuse together forming carbon-12 in a two-stage process
as three helium nuclei(ALPHA PARTICLES) are used it is called the TRIPLE ALPHA PROCESS
subsequent fusion with anther helium nucleus produces oxygen-16 nuclei. this process is the main source of the carbon and oxygen found in the universe, including that in our bodies
the horizontal branch
have a helium core-burning and hydrogen shell-burning
a solar-mass star has sufficient helium fuel for the core-burning to last for abut 100 million years
the asymptotic giant branch
energy from the helium-burning in turn heats up surrounding unused hydrogen which also starts shell burning
the giant stars expands again, possibly up to 1.5 AU equivalent to the orbit of the mars
occupying the upper-right portion of the HR diagram
exp: MIRA (O CETI)
1 solar mass AGB may have a luminosity 10 000 * that of our current sun
often more luminous in the infrared than visible wavelength
intrinsic variable stars with periods of months or a few years
nuclear reactions
proton-proton chain
CNO cycle
triple alpha